J. Sadler

Adaptive beamforming for ultrasonic phased array focusing through layered structures

Successful realization of ultrasonic imaging through a multilayered composite barrier is hampered by scattering, attenuation, and multiple reflections of acoustic waves at and inside the barrier. These effects tend to distort the beam pattern produced by conventional phased arrays, defocusing the ultrasonic field transmitted through the barrier and causing image quality degradation and resolution loss. To compensate for the refraction and multiple reflection effects, we developed an adaptive beamforming algorithm for small-aperture linear phased arrays. After assessing the barriers local geometry, the method calculates a new timing distribution to refocus the distorted beam at its original location. The procedure is in fact a construction of a matched filter that automatically adapts the transmission pattern of the phased array to the local geometry of the barrier and cancel its distorting effect In this work, the adaptive beamforming algorithms, in transmission mode, for the barriers in the form of a flat homogeneous layer, a layer with a smooth, randomly curved back surface and a two-layered combination of the above have been developed and experimentally verified on custom-engineered samples with prescribed acoustical properties. The algorithms were implemented on ULA-OP, an ultrasound advanced open-platform (University of Florence), controlling 64 active elements on a 128-elements phased array. Experimental measurements of original, distorted and corrected beam profiles confirm the ability of our algorithms to refocus the beam after passing through a scattering and refractive sample. Different excitation signals and windowing options introduced through ULA-OP were examined and compared.

4I-2 Ultrasonic Pulse-Echo NDE of Adhesive Bonds in Sheet-Metal Assemblies

The development of a 20 MHz pulse-echo method for nondestructive evaluation (NDE) of adhesive bonds was undertaken to provide assurance of bond integrity in vehicle body assemblies. This new NDE method features improvements over previous methods implemented in production, and extends the range of bond evaluation effectiveness. The NDE is accomplished by the acquisition and analysis of acoustic echoes that return from bond joints that have interfaces between layers with large acoustical impedance mismatch. These echoes reverberate in the multilayered joint structures and are captured for a computer-automated analysis that provides a rapid interpretation of the indications, and subsequently yields a simplified display of the inspection results

P3K-4 Utilizing a Ray Technique to Calculate Multiple Reflections and Transmitted Waves from Layered Plates and Examine the Detection of Critical Disbonds, and Lamb Modes Generated

This paper considers the problem of a single, or multilayer, isotropic plate and the problem of calculating the multiple reflections of acoustic waves within the plate, and the acoustic waves transmitted outside the plate (leaky waves). By using a ray method, and considering the paths traveled by the rays, it has been found that many of the multiple reflections will travel equivalent paths in space and time. Using superposition to combine the equivalent paths reduces the number of rays to track, and allows the process of calculations to be done in a time efficient manner. The use of the a ray model also gives initial results which are independent of properties of the incident acoustic waves. A separate set of calculations can be used expand the rays to a variety of more realistic acoustic waves, allowing a single result to be used for a variety of acoustic sources. The results from this model will be used to examine situations such as the detection of critical disbonds in plates, examination of the lamb modes which are generated, or examination of which situations produce the largest transmitted waves

Ultrasonic Imaging of Foreign Inclusions and Blood Vessels Through Thick Skull Bones

We report a new progress in the development of a portable ultrasonic transcranial imaging system, which is expected to significantly improve the clinical utility of transcranial diagnostic ultrasound. When conventional ultrasonic phased array and Doppler techniques are applied through thick skull bones, the ultrasound field is attenuated, deflected, and defocused, leading to image distortion. To address these deficiencies, the ultrasonic transcranial imaging system implements two alternative ultrasonic methods. The first method improves detection of small foreign objects, such as bone fragments, pieces of shrapnel, or bullets, lodged in the brain tissue. Using adaptive beamforming, the method compensates for phase aberration induced by the skull and refocuses the distorted ultrasonic field at the desired location. The second method visualizes the blood flow through intact human skull using ultrasonic speckle reflections from the blood cells, platelets, or contrast agents. By analyzing these random temporal changes, it is possible to obtain 2D or 3D blood flow images, despite the adverse influence of the skull. Both methods were implemented on an advanced open platform phased array controller driving linear and matrix array probes. They were tested on realistic skull bone and head phantoms with foreign inclusions and blood vessel models.

Transmission mode adaptive beamforming for planar phased arrays and its application to 3D ultrasonic transcranial imaging

A new adaptive beamforming method for accurately focusing ultrasound behind highly scattering layers of human skull and its application to 3D transcranial imaging via small-aperture planar phased arrays are reported. Due to its undulating, inhomogeneous, porous, and highly attenuative structure, human skull bone severely distorts ultrasonic beams produced by conventional focusing methods in both imaging and therapeutic applications. Strong acoustical mismatch between the skull and brain tissues, in addition to the skulls undulating topology across the active area of a planar ultrasonic probe, could cause multiple reflections and unpredictable refraction during beamforming and imaging processes. Such effects could significantly deflect the probes beam from the intended focal point. Presented here is a theoretical basis and simulation results of an adaptive beamforming method that compensates for the latter effects in transmission mode, accompanied by experimental verification. The probe is a custom-designed 2 MHz, 256-element matrix array with 0.45 mm element size and 0.1mm kerf. Through its small footprint, it is possible to accurately measure the profile of the skull segment in contact with the probe and feed the results into our ray tracing program. The latter calculates the new time delay patterns adapted to the geometrical and acoustical properties of the skull phantom segment in contact with the probe. The time delay patterns correct for the refraction at the skull-brain boundary and bring the distorted beam back to its intended focus. The algorithms were implemented on the ultrasound open-platform ULA-OP (developed at the University of Florence).

Acoustic Microscopy Study of Properties and Microstructure of Synthetic and Natural Fiber Composite Materials

Bio-fiber based composites are gaining acceptance in today’s industries. Aimed to replace synthetic fiber-reinforced composite materials, natural fiber composites significantly differ from them in microstructure. Development of new bio-composites shows necessity for effective studies of their mechanical properties. In this paper, properties of bio-composites with different content of wheat straw fillers are analyzed. Microstructure of the composites, its relation to material composition and distribution of the filler phase in the polymer matrix is investigated with acoustic microscopy. The obtained results help to deeper understand structure-property relationship for bio-composite materials and provide feedback for development of new materials with properties satisfactory for industrial needs.

High-resolution blood flow imaging through the skull

Blood flow imaging in brain and diagnosis of injuries under the skull remains a challenging problem since in most emergency cases such as a battlefield, ambulance or clinic, there is no MRI or CT scanner to make an urgent decision. In this paper we show that not only the frequency contents of the ultrasound signals but also the resonance components of ultrasound A-scans may need to be taken into consideration to construct high-resolution blood flow images through the skull using ultrasound. Several experiments have been carried out using various custom skull phantoms to match the ultrasonic properties of the human skull including a diploe layer and undulating inner surface. The results indicate a significant improvement in terms of accuracy and computational cost.

12D-5 NDE of Adhesive Joints Using V(x,t) Data

In this paper we examine the NDE problem of the detection of bonding, verses disbonding, at the interfaces of a typical bonded plate. By using a wide angle focused transducers results are obtained in the space-time domain (V(x,t) data). This setup generates a variety of Lamb modes within the plate, provides leaky wave data spread throughout the time space domain, and eliminates the time domain overlap problems of waveforms from normal incidence testing method. Directly comparing the V(x,t) data additional leaky mode reflections are observed, where the period of these reflections if found to increase linearly with the thickness of the adhesive layer. Additionally we investigate the K-F map (V(k,f) data) generated from the 2D Fourier transform of the V(x,t) data, and briefly compare the expected theoretical results with the experimental results for both V(x,t) data, and K-F maps.

P0-10 A Ray Technique to Calculate Multiple Reflected and Transmitted Waves in Layered Media

In this paper we consider the problem of calculating the propagation of acoustic waves within an ideal solid isotropic multilayer plate structure. In such a situation the process of mode conversion as the wave interacts with each interface of the plate creates an ever increasing number of waves to track, and to perform calculations on, as the wave propagates within the layered media. We choose to explore this problem by examining the ray paths of the multiple reflections within the plate structure, and show that upon careful consideration many of these paths will travel equivalent distances in time and space becoming coincident. The principle of superposition can then be used to combine these coincident paths, this reduces the number of waves to track, and simplifies the problem so that the necessary calculations can be performed in a time efficient manner.